The cell biology of both Leishmania and Toxoplasma are investigated as models of intracellular parasitism. Emphasis is placed on characterizing the biochemical and physiological functions of their surface membranes (SM) and secreted components toward defining the roles of these constituents in parasite survival and development. Both Leishmania and Crithidia were shown to methylate surface membrane (SM) proteins in response to environmental stress. Cumulative quantitative data indicate that these organisms can adapt to environmental changes using methylation as a means to rapidly modify/regulate SM protein functions in situ. Over 800 nucleotides of a genomic clone of an L. donovani secretory acid phosphatase has been sequenced, and it is being used as a probe to obtain a full length cDNA copy of this gene. Experiments are also in progress toward cloning the gene for the unique trypanosomatid SM enzyme, 3'-nucleotidase. Methods were devised for the continuous in vitro cultivation of large quantities of infectious amastigotes from L. donovani and five other Leishmania species. These are being characterized with regard to their cell biology, SM-biochemistry and physiology. Further, at the molecular level, genes uniquely expressed by these amastigotes are being used as probes to study parasite gene regulated differentiation and development. In addition, studies were initiated concerning the functional SM biochemistry of Toxoplasma gondii. The current results are of relevance toward the development of improved diagnostic, chemotherapeutic and immunoprophylactic agents against these important human pathogens.